Communication systems are known to comprise communication units, such as in-car mobile or hand-held portable radios, as well as a fixed infrastructure, such as base stations and/or controllers. A typical message within such a communication system may begin with a communication unit converting an audio signal into a digital data stream suitable for transmission over an RF (radio frequency) channel to either another communication unit or the fixed infrastructure. Such systems are often used by public safety institutions, such as local or federal law enforcement agencies. The existence of commercially available RF scanners makes it possible for unauthorized parties to monitor the information transmitted within such a communication system. To reduce unauthorized eavesdropping, communication systems encrypt communications such that, without knowledge of the encryption method and a decryptor, the communications are unintelligible.
As is known, digital encryption methods use a reversible algorithm to introduce randomness into a digital data stream. An algorithm that randomizes digital data is called an encryptor; that which reconstructs the original data from the randomized data, a decryptor. An encryptor/decryptor algorithm typically utilizes dynamic parameters, hereafter referred to as keys, to uniquely specify the nature of the randomness introduced to the digital data stream. Thus, only encryptors and decryptors utilizing an identical algorithm and key are capable of communicating intelligible messages.
It is often the case that talkgroups (i.e., a group of logically related communication units configured to receive communications intended for the entire group) are partitioned by key variables on the same channel. For example, if a first talkgroup is partitioned through the use of a first key on a given channel and a second talkgroup is partitioned through the use of a second key on the same channel, encrypted messages intended for the first talkgroup (i.e., messages encrypted with the first encryption key) will be correctly decrypted by communication units within the first talkgroup. In the second talkgroup, however, communication units utilizing the second key will attempt to decrypt the message, resulting in digital streams of unintelligible data. Unless provided a method for detecting the key mismatch, communication units in the second talkgroup will render the unintelligible data audible to their respective users, often resulting in annoyed users.
Prior art solutions to this problem have relied upon the assumption that certain bit patterns are prevalent in digitally represented speech signals. For example, digitized audio signals created through the use of a CVSD (Continuously-Variable Slope-Delta) vocoder are assumed to include significant amounts of idle pattern (i.e., 1010 . . . ), alternating one-zero pairs (i.e., 1100 . . . ), and long one/zero runs (i.e., 11111010000010 . . . ). In these methods, correlations are performed between the decrypted digital data and the desired bit patterns. If there is a high degree of correlation between the decrypted digital data and the desired bit patterns, it is assumed that the message has been correctly decrypted (i.e., the correct key has been used), and the resulting audio is unmuted for presentation to the user. If the degree of correlation is insufficient, the resulting audio is muted.
The previously described methods suffer the shortcoming of being overly strict. That is, they often cause messages that have been correctly decrypted to be muted nonetheless. This is a result of intelligible speech signals that do not contain significant amounts of the desired bit patterns, i.e., speech modulated with high-level background noise. As a result of this shortcoming, it is possible for users to miss entire messages. Therefore, a need currently exists for a method of reliably detecting correctly decrypted communications that overcomes the shortcomings of prior art solutions.